Acta Orthopædica Belgica, Vol. 82 - 1 - 2016 The infrapatellar (Hoffa’s) fatpad is an important structure within the knee, whose function and role are both poorly understood. This review explores the anatomy, neural innervation, vascularity, role in bio- mechanics, pathology, imaging (stressing the impor- tance of dynamic ultrasound assessment) and treat- ment of disorders presenting within this structure. Keywords : infrapatellar fatpad ; Hoffa’s fatpad ; ultra- sound ; fatpad impingement. INTRODUCTION In 1904, Albert Hoffa first attributed impinge- ment of the Infrapatellar fat pad to symptoms of knee pain. He described inflammatory fibrous hy- perplasia of the infrapatellar fat pad at excision, a procedure resulting in symptomatic relief in his se- ries of 21 patients (29). In the last 109 years there still remains an apparent paucity of understanding in the role of the Infrapatellar fat pad in health and disease (10,11,21,24). Hoffa disease has been defined as, “an impingement of the hypertrophic fat pad be- tween the articular surfaces of the knee (femeropa- tellar and femerotibial)” it has also been differenti- ated from Hoffa syndrome, the former being fat fad oedema and fibrosis in a normal joint, with the latter occurring in conjunction with concomitant pathol- ogy (43). Reviewing current literature yields at best subjective clinical observations and somewhat small case series (3,14,15,17,19-21,26,29,35,38,39,43,46, 47). We present an overview of information pertain- ing to the Infrapatellar fat pad and the role it may play in pathology and pain around the knee. Anatomy The infrapatellar fat pad is one of three fat pads located in the anterior aspect of the knee (24). Its macroscopic, arthroscopic and radiographic ana- tomic boundaries are all well described (7,11,21,24,32, 37,43,49,50). Gallagher et al detail cadaveric anato- my, finding it to be a constant structure bounded superiorly by the inferior pole of the patella, inferi- orly by the anterior tibia, intermeniscal ligament, meniscal horns and infrapatellar bursa, anteriorly by the patellar tendon and posteriorly by the femoral Conflicts of interests : Mr S. Anand holds an educational contract with Smith and Nephew. Acta Orthop. Belg., 2016, 82, 94-101 Infrapatellar fat pad syndrome : a review of anatomy, function, treatment and dynamics Mr James MACE, Prof Waqar BHATTI, Mr Sanjay ANAND From the Department of Trauma and Orthopaedics, Stepping Hill hospital NHS Foundation Trust, Cheshire, UK ORIGINAL STUDY n Mr James Mace. n Mr. Sanjay Anand. Department of Trauma and Orthopaedics, Stepping Hill hospital NHS Foundation Trust, Poplar Grove, Hazel Grove, Stockport, Cheshire, UK. n Prof. Waqar Bhatti. Department of Radiology, University Hospital of South Manchester, Southmoor road, Wythenshawe, Manchester, M23 9LT, UK. Correspondence : Mr James Mace, 27 Vaudrey Drive, Cheadle Hulme, Cheadle Cheshire SK8 5LR, UK. E-mail : [email protected] © 2016, Acta Orthopædica Belgica.
Infrapatellar fat pad syndrome : a review of anatomy, function, treatment and dynamics
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Acta Orthopædica Belgica, Vol. 82 - 1 - 2016Acta Orthopædica Belgica, Vol. 82 - 1 - 2016
The infrapatellar (Hoffa’s) fatpad is an important structure within the knee, whose function and role are both poorly understood. This review explores the anatomy, neural innervation, vascularity, role in bio- mechanics, pathology, imaging (stressing the impor- tance of dynamic ultrasound assessment) and treat- ment of disorders presenting within this structure.
Keywords : infrapatellar fatpad ; Hoffa’s fatpad ; ultra- sound ; fatpad impingement.
INTRODUCTION
In 1904, Albert Hoffa first attributed impinge- ment of the Infrapatellar fat pad to symptoms of knee pain. He described inflammatory fibrous hy- perplasia of the infrapatellar fat pad at excision, a procedure resulting in symptomatic relief in his se- ries of 21 patients (29). In the last 109 years there still remains an apparent paucity of understanding in the role of the Infrapatellar fat pad in health and disease (10,11,21,24). Hoffa disease has been defined as, “an impingement of the hypertrophic fat pad be- tween the articular surfaces of the knee (femeropa- tellar and femerotibial)” it has also been differenti- ated from Hoffa syndrome, the former being fat fad oedema and fibrosis in a normal joint, with the latter occurring in conjunction with concomitant pathol- ogy (43). Reviewing current literature yields at best subjective clinical observations and somewhat
small case series (3,14,15,17,19-21,26,29,35,38,39,43,46, 47). We present an overview of information pertain- ing to the Infrapatellar fat pad and the role it may play in pathology and pain around the knee.
Anatomy
The infrapatellar fat pad is one of three fat pads located in the anterior aspect of the knee (24). Its macroscopic, arthroscopic and radiographic ana- tomic boundaries are all well described (7,11,21,24,32, 37,43,49,50). Gallagher et al detail cadaveric anato- my, finding it to be a constant structure bounded superiorly by the inferior pole of the patella, inferi- orly by the anterior tibia, intermeniscal ligament, meniscal horns and infrapatellar bursa, anteriorly by the patellar tendon and posteriorly by the femoral
Conflicts of interests : Mr S. Anand holds an educational contract with Smith and Nephew.
Acta Orthop. Belg., 2016, 82, 94-101
Infrapatellar fat pad syndrome : a review of anatomy, function, treatment and dynamics
Mr James Mace, Prof Waqar Bhatti, Mr Sanjay anand
From the Department of Trauma and Orthopaedics, Stepping Hill hospital NHS Foundation Trust, Cheshire, UK
ORIGINAL STUDY
n Mr James Mace. n Mr. Sanjay Anand. Department of Trauma and Orthopaedics, Stepping Hill
hospital NHS Foundation Trust, Poplar Grove, Hazel Grove, Stockport, Cheshire, UK.
n Prof. Waqar Bhatti. Department of Radiology, University Hospital of South
Manchester, Southmoor road, Wythenshawe, Manchester, M23 9LT, UK. Correspondence : Mr James Mace, 27 Vaudrey Drive,
Cheadle Hulme, Cheadle Cheshire SK8 5LR, UK. E-mail : [email protected] © 2016, Acta Orthopædica Belgica.
mace-.indd 94 10/03/16 11:22
infrapatellar fat pad syndroMe 95
condyles and intercondylar notch. Its attachments are not only to the intercondylar notch via the liga- mentum mucosum, but also into the anterior horns of the menisci, the proximal end of the patella ten- don and the inferior pole of the patella (24). Lying intra-articular but extra synovial and occupying the whole anterior part of the knee joint in all joint posi- tions (10) it consists of a central body with medial and lateral extensions, along with a superior tag, the latter of which is not always an anatomical con- stant (24). There are 2 clefts identified macroscopi- cally and, especially in the presence of an effusion, via MR imaging ; one vertical cleft in the superior aspect of the fat pad and horizontal cleft in the postroinferior aspect of the fat pad (1,12,24,32,50,53). During knee trauma the fatpad may fragment mim- icking loose bodies on MRI imaging, or hide true loose bodies within its clefts at arthroscopy (2).
Vascularity
For a structure to share morphological similari- ties with subcutaneous fat (50), yet only be metabo- lised in severe malnutrition, and not expand with increasing BMI (16,24,29,50), implies a degree of bio- logical significance (10). It possesses an abundant peripheral anastamotic blood supply, the suprome- dial and suprolateral geniculate arteries provide 2 vertical arteries and are linked horizontally with 2-3 arteries running distally (37). In addition, there are rich local anastamotic links to the menisci anterior- ly, the tibial periostium inferiroly and to the patellar tendon anteriorly. Anatomic studies by Pang et al (48) revealed a large, mostly transversely orien- tated, contribution of vessels to the posterior aspect and central third of the patellar tendon. The central area is relatively avascular (37), having connotations for central arthroscopy portals and in the harvest of the fatpad for reconstructive measures.
Three in vivo studies using Laser Doppler Flow- metry (LDF) have sought to monitor blood flow to the patellar during surgical approach for total knee arthroplasty. Although recently no significant cor- relation between LDF and post operative anterior knee pain has been found (36), these studies still re- veal useful information pertaining to the shared vas- cular supply of the infrapatellar fat pad in clinical practice and the contribution of the anastamotic vas-
cular supply to surrounding structures. Hughes et al demonstrated a reduction in patellar blood flow of 10% following resection of the inftapatellar fat pad (31). Nicholls et al did not resect the fat pad in their study, believing it to be an important contribu- tor to vascularity of the patellar tendon (45), they found no significant difference in decline in flow to the patellar from a medial or lateral arthrotomy, supporting the notion of a rich functional anastamo- sis. Hempfing et al found an increase in patellar blood flow after excision of the fatpad, with the knee in extension (27), they postulated the excision of the fatpad decompressed the patellar in extension allowing for an increased blood flow. The varying techniques and differing equipment used through- out these 3 studies makes direct comparisons im- possible, but do suggest the infrapatellar fatpad makes some contribution to patellar blood flow.
Neural innervation
The nerve supply to the knee has been histori- cally well documented firstly by Garder and latterly Kennedy et al (25,34), with the knee taking innerva- tions from the femoral, obturator and sciatic nerves. The predominant nerve supply to the fatpad is the posterior tibial nerve (34), which provides the ma- jority of fibres to the popliteal plexus ; fibres course from this innervating the posterior capsule, cruci- ates and anteriorly up to the fatpad.
A superfluity of Type VIa free nerve endings has been identified within the fatpad (6,10,24,50) ; these Type VIa free nerve endings can be activated by mechanical deformation or specific immunoreac- tive chemical agents. They initiate afferent signals of pain, pressure and thermal changes to the central nervous system and are active via both fast sharp pain pathways and slow chronic pain pathways (6). Dye et al (22) describe the conscious neuorsensory perception of pain on arthroscopically probing structures of the knee without anaesthetic ; finding the infrapatellar fat pad capable of triggering both severe and localising pain.
Substance-P has been implicated in the genesis of both pain and the induction of a pro-inflammatory response ; several authors have found substance-P nerve fibres in abundance within the fat pad (9,40,55). Furthermore in patients experiencing anterior knee
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Acta Orthopædica Belgica, Vol. 82 - 1 - 2016
pain the number of substance –P nerve fibres within the fatpad increases significantly (40,54). The pres- ence and number of nociceptive afferent immunore- active nerve fibres and ability to release pro-inflam- matory cytokines has led to some question of the fat pad being a potential causative structure in patellar tendinopathy (6,16). There are suggestions it per- forms a modulatory role in the inflammatory path- ways active in osteoarthritis (13) and in an in vitro bovine cartilage model the infrapatellar fat pad tis- sue has been demonstrated to reduce catabolic me- diators (3).
The role of infrapatellar fat pad in biomechanics
In 1950 MacConail surmised the role of fat pads in joints is in occupying anatomical dead space to allow synovial fluid to both circulate around a joint and provide efficient lubrication (41), the rich neuro- vascular supply of the infrapatellar fatpad suggests this may be a simplistic view. A correlation between weight of the infrapatellar fat pad and body height has been demonstrated, perhaps inferring a biome- chanical role within the knee joint (18). Given the rich nociceptive content of the inftapatellar fat pad it comes as no surprise that a form of anterior knee pain can be reliably evoked in normal patients through injection of hypertonic saline into the fat- pad (4,5,28). Patients suffering with anterior knee pain have been demonstrated to show a diminished coordination of motor units between medial and lateral vastus muscle units (44). Hodges et al (28) concluded that after a painful hypertonic saline in- jection to the infrapatellar fat pad there was a sig- nificantly later activation and reduced amplitude of contraction of quadriceps during stair stepping. This deactivation of the quadriceps has been assumed to increase patellofemoral loading leading to an in- creased incidence of patellofemoral cartilage de- generation in some reports of Hoffa’s disease (39).
Bohnsack et al (10), using a cadaveric model, found resection of the infrapatellar fat pad reduced patellofemoral contact pressures, produced a sig- nificant medialisation of the patella and decreased tibial rotation during flexion. Using the same model in a later study Bohnsack et al (8) sought to simulate oedema and mass effect of the fat pad using an in-
flatable fluid cell implanted within the fat pad. An increase in simulated oedema again decreased patel- lofemoral contact area and pressure, whilst increas- ing measured pressure within the fatpad. However, although these cadaveric studies confer a role of the fatpad in biomechanics of the knee, they may not represent true in vivo findings.
Two studies have sought to link the presence of oedema within the fat pad to patellofemoral mal- tracking (33,52), both support the view that oedema within the suprolateral portion of the infrapatellar fat pad is linked with radiological features sugges- tive of patella maltracking, especially an increase in the prevalence of patella alta, the exact cause of this remains unclear.
Diagnosis (inc. prevalence) of fat pad pathology
The diagnosis of infrapatellar fat pad impinge- ment is often considered a rare diagnosis of exclu- sion, or misdiagnosed in cases of recalcitrant ante- rior knee pain (39,43). The population prevalence is not known ; Smillie, in 1962, latterly supported by subsequent authors, separated the rarer primary hy- pertrophy and impingement from the more common secondary disease, occurring in the presence of oth- er pathology such as meniscal tears and ligemen- tous injuries (39,43,51). Kumar et al report a series of 2623 patients undergoing knee arthroscopy, finding its presence as an isolated (primary) lesion in 34 (1.3%) of patients and coexisting with other pathol- ogies as secondary disease in 178 cases (6.8%). Ogilvie-Haris and Giddens report incidence of 1% of patients over a ten year period containing around 1200 patients undergoing surgery.
Several authors have outlined clinical features and diagnostic aids. Anterior knee pain, typically felt in the retropatellar and infrapatellar regions can be considered a universal presentation. There may be an association with patellofemoral crepitus, with some reporting loading the knee and in particular ascending and descending stairs precipitating pain (11,21,38,39,43,47,53).
The importance of excluding causes of other pa- thology by careful examination in particular of the hip and spine should be accepted. Detailed exami- nation of the knee should be performed to locate
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awake patient will not fully extend their knees to the point impingement occurs (39). During a pilot study for ultrasound guided alcohol ablation of the in- frapatellar fat pad House and Connell (30) noted pronounced sonographic anomalies in consisting of hypoechogenicity and neovascularity on colour Doppler. The widespread validity in the use of ul- trasound in diagnosis is still unknown. We have tended to note an relationship of suprolateral fatpad impingement in association with a perceived tight Iliotibial band and through dynamic sonographic assessment have noted a correlation with lateral pa- tella compression syndrome ; but stress that any part of the fatpad can be responsible for impinge- ment. In addition we have observed cases of pre- femoral fatpad impingement presenting with similar symptoms (Fig. 2).
Diagnostic and therapeutic injections of local an- aesthetic and steroid into the fat pad have been rec- ommended by some (21). In these cases it is reported
maximal area of discomfort. The inrapatellar fat pad may appear enlarged and firm in consistency to pal- pation (21,29). Compression of the patellofemoral joint may also cause pain (39,43) Hoffa’s test can be performed and is well described. It is performed with the hips and knees both flexed to 90 degrees ; pressure is then applied to the medial and lateral joint lines. The test is positive for impingement if pain is produced during the last 10 degrees of exten- sion (39,43,47). Kumar et al suggested a modification of this test ; stating the fat pad should not be pal- pated during the test as pressure around the joint could trigger pain in surrounding structures giving rise to false positive results. Instead they recom- mend a passive forced hyperextension by lifting the heel keeping pressure on the anterior tibia, believ- ing if the fat pad is pathological pain will be repro- duced without any direct pressure (39). No figures for the sensitivity or specificity of Hoffa’s test, or its modifications could be found in the construction of this review.
Synovial inflammatory and proliferative causes should be actively excluded through history and the relevant haematological, biochemical and immuno- logical investigations (39,47). Radiographic assess- ment of the knee should be undertaken as needed, the acquisition of basic roentgenographic examina- tion is well supported (23,39,43,47). The role for fur- ther imaging remains controversial. Most authors recommend further imaging, usually in the form of MRI (although CT and radio nucleotide scans have also been advocated (43)). Von Engelhardt et al un- dertook preoperative MRI scans in 62 patients with clinically suspected and arthroscopically confirmed secondary infrapatellar fatpad impingement. Whilst there was no single pathognominic features they found some associations present statistically signifi- cantly more often than in case controlled MRI scans in patients without impingement. These changes in- cluded oedema of the superior/posterior fat pad (present up in up to 48% vs 27% of non impinge- ment control) and the presence of an inflamed in- frapatellar bursa (present in up to 66% of patients with impingement and up to 43% of patients with- out) (53). Kumar et al refute the need for routine MRI scans unless additional pathology is suspected, stating that impingement will not be visible as an
Fig. 1. — T2 weighted MRI scan in coronal plane illustrating typical appearances of suprolateral fatpad oedema from impingement
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Acta Orthopædica Belgica, Vol. 82 - 1 - 2016
incarcerated between the patella tendon and anterior femur, often creating deformation of the tendon in doing so, and again associated with the onset of pain.
Findings at arthroscopy
Aside from Hoffa’s original report in 1902, most authors recommend arthroscopically treating im- pingement of the infrapatellar fat pad. Due to the more common occurrence in conjunction with other pathology (21,39) a careful and thorough arthroscop- ic evaluation of the whole joint should be per- formed. Portals described for adequate visualisation and access are : antrolateral, antromedial, high an- trolateral, high antromedial, midpatella lateral and high antromedial (39,43,47). The typical arthroscopic findings are of a hypertrophic, firm, inflamed fat- pad. Often signs of fibrosis will be present with a whitish covering instead of the normal yellow ap- pearances. There may be adhesions present and vil- lous changes to the overlying synovium (39,43,47,53). The impingement lesion should be visualised with the knee fully extended and the knee deflated (39). Arthroscopic shavers are the instrument of choice for resection, which is carried out cautiously to pro- tect integrity of the patellar tendon. Kumar et al (39) describe three separate arthroscopic morphologies, the first pertained to acute injury related disease with duration of less that 4 months (defined as type 1) ; the fatpad appeared inflamed with contused vil- lous extensions encroaching over the anterior horns of the menisci, engorged blood vessels and signs of recent haemorrhage may be visible. In the more chronic cases (with and without fibrosis – classified as type 2 and 3 respectively) changes included less contusion, but indentation of hyaline cartilage over the femoral condyle at the point of abutment in full extension (type 2), progressing to a firmer fibrotic fatpad with occasional areas of calcification or car- tilaginous degeneration (type 3).
Histology in infrapatellar fat pad impingement
Biopsy at time of surgery is recommended to confirm diagnosis and if there is any doubt of the presence of impingement normal histology should prevent excision (47). Histology will tend to show
an immediate relief of pain and restoration of move- ment. There is a strong consensus of recommending a course of physiotherapy prior to proceeding to op- erative intervention (39,43,47).
Fat pad Dynamics
We believe fat pad impingement to be a dynamic phenomenon that is difficult to fully appreciate through the medium of static imaging. Using high resolution ultrasound we have begun to explore fat- pad kinesiology and have noted varying sites of im- pingement in both knee flexion and extension.
The superolateral portion of the fatpad can be noted to adopt a relaxed, freely expansive and rela- tively fluid state in flexion. On moving to extension the morphology changes as the fatpad becomes in- carcerated by the lateral patella facet and quadriceps tendon ; in symptomatic patients the onset of symp- toms follows this process ; which we have observed to be the most common site of impingement. There may follow an association with the lateral patellar compression syndrome.
The less commonly seen infrapatella fatpad im- pingement follows an opposite pattern, becoming
Fig. 2. — T2 weighted MRI scan in coronal plane illustrating typical appearances of prefemoral fatpad oedema from impingement.
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section on pain after total knee replacement. They found statistically significantly more patients were pain free at 6 months if the fat pad was removed op- posed to retained.
CONCLUSION
Despite being a 110 year old condition the mech- anisms involved in pathogenesis of pain from the infrapatellar fat pad are still largely misunderstood. What is clear from current literature is the complex- ity of the structure and its assessment. If infrapatel- lar impingement is diagnosed correctly, treatment from resection will see improvement in symptoms of pain and function. Further prospective, controlled trials are warranted in the assessment of the roles of surgery, chemical ablation and steroid injections. The indications and value for ultrasonographic dynamic assessment is still in the infancy of wide- spread clinical adoption, but is a technique we have found to be of great assistance and continue to develop.
dilated vessels, synovial hypertrophy, fibrosis or chronic hypertrophy occasionally progressing to calcification or transformation to fibrocartilaginous tissue (21,39,43). Fibrosis and vascular neoformation has been shown to be present in the majority of pa- tients undergoing total knee replacement, with chronic inflammatory changes present in over a third (42), perhaps reflecting the complex involve- ment of the fatpad in the presence of intra articular pathology (13).
RESULTS OF CURRENT STUDIES
There are currently five observational studies without control (level 4 evidence) describing treat- ment of infrapatellar impingement in the recent literature, the findings of these are summarised in table I.
The disparity of data presented from the above studies renders aggregation difficult,…
The infrapatellar (Hoffa’s) fatpad is an important structure within the knee, whose function and role are both poorly understood. This review explores the anatomy, neural innervation, vascularity, role in bio- mechanics, pathology, imaging (stressing the impor- tance of dynamic ultrasound assessment) and treat- ment of disorders presenting within this structure.
Keywords : infrapatellar fatpad ; Hoffa’s fatpad ; ultra- sound ; fatpad impingement.
INTRODUCTION
In 1904, Albert Hoffa first attributed impinge- ment of the Infrapatellar fat pad to symptoms of knee pain. He described inflammatory fibrous hy- perplasia of the infrapatellar fat pad at excision, a procedure resulting in symptomatic relief in his se- ries of 21 patients (29). In the last 109 years there still remains an apparent paucity of understanding in the role of the Infrapatellar fat pad in health and disease (10,11,21,24). Hoffa disease has been defined as, “an impingement of the hypertrophic fat pad be- tween the articular surfaces of the knee (femeropa- tellar and femerotibial)” it has also been differenti- ated from Hoffa syndrome, the former being fat fad oedema and fibrosis in a normal joint, with the latter occurring in conjunction with concomitant pathol- ogy (43). Reviewing current literature yields at best subjective clinical observations and somewhat
small case series (3,14,15,17,19-21,26,29,35,38,39,43,46, 47). We present an overview of information pertain- ing to the Infrapatellar fat pad and the role it may play in pathology and pain around the knee.
Anatomy
The infrapatellar fat pad is one of three fat pads located in the anterior aspect of the knee (24). Its macroscopic, arthroscopic and radiographic ana- tomic boundaries are all well described (7,11,21,24,32, 37,43,49,50). Gallagher et al detail cadaveric anato- my, finding it to be a constant structure bounded superiorly by the inferior pole of the patella, inferi- orly by the anterior tibia, intermeniscal ligament, meniscal horns and infrapatellar bursa, anteriorly by the patellar tendon and posteriorly by the femoral
Conflicts of interests : Mr S. Anand holds an educational contract with Smith and Nephew.
Acta Orthop. Belg., 2016, 82, 94-101
Infrapatellar fat pad syndrome : a review of anatomy, function, treatment and dynamics
Mr James Mace, Prof Waqar Bhatti, Mr Sanjay anand
From the Department of Trauma and Orthopaedics, Stepping Hill hospital NHS Foundation Trust, Cheshire, UK
ORIGINAL STUDY
n Mr James Mace. n Mr. Sanjay Anand. Department of Trauma and Orthopaedics, Stepping Hill
hospital NHS Foundation Trust, Poplar Grove, Hazel Grove, Stockport, Cheshire, UK.
n Prof. Waqar Bhatti. Department of Radiology, University Hospital of South
Manchester, Southmoor road, Wythenshawe, Manchester, M23 9LT, UK. Correspondence : Mr James Mace, 27 Vaudrey Drive,
Cheadle Hulme, Cheadle Cheshire SK8 5LR, UK. E-mail : [email protected] © 2016, Acta Orthopædica Belgica.
mace-.indd 94 10/03/16 11:22
infrapatellar fat pad syndroMe 95
condyles and intercondylar notch. Its attachments are not only to the intercondylar notch via the liga- mentum mucosum, but also into the anterior horns of the menisci, the proximal end of the patella ten- don and the inferior pole of the patella (24). Lying intra-articular but extra synovial and occupying the whole anterior part of the knee joint in all joint posi- tions (10) it consists of a central body with medial and lateral extensions, along with a superior tag, the latter of which is not always an anatomical con- stant (24). There are 2 clefts identified macroscopi- cally and, especially in the presence of an effusion, via MR imaging ; one vertical cleft in the superior aspect of the fat pad and horizontal cleft in the postroinferior aspect of the fat pad (1,12,24,32,50,53). During knee trauma the fatpad may fragment mim- icking loose bodies on MRI imaging, or hide true loose bodies within its clefts at arthroscopy (2).
Vascularity
For a structure to share morphological similari- ties with subcutaneous fat (50), yet only be metabo- lised in severe malnutrition, and not expand with increasing BMI (16,24,29,50), implies a degree of bio- logical significance (10). It possesses an abundant peripheral anastamotic blood supply, the suprome- dial and suprolateral geniculate arteries provide 2 vertical arteries and are linked horizontally with 2-3 arteries running distally (37). In addition, there are rich local anastamotic links to the menisci anterior- ly, the tibial periostium inferiroly and to the patellar tendon anteriorly. Anatomic studies by Pang et al (48) revealed a large, mostly transversely orien- tated, contribution of vessels to the posterior aspect and central third of the patellar tendon. The central area is relatively avascular (37), having connotations for central arthroscopy portals and in the harvest of the fatpad for reconstructive measures.
Three in vivo studies using Laser Doppler Flow- metry (LDF) have sought to monitor blood flow to the patellar during surgical approach for total knee arthroplasty. Although recently no significant cor- relation between LDF and post operative anterior knee pain has been found (36), these studies still re- veal useful information pertaining to the shared vas- cular supply of the infrapatellar fat pad in clinical practice and the contribution of the anastamotic vas-
cular supply to surrounding structures. Hughes et al demonstrated a reduction in patellar blood flow of 10% following resection of the inftapatellar fat pad (31). Nicholls et al did not resect the fat pad in their study, believing it to be an important contribu- tor to vascularity of the patellar tendon (45), they found no significant difference in decline in flow to the patellar from a medial or lateral arthrotomy, supporting the notion of a rich functional anastamo- sis. Hempfing et al found an increase in patellar blood flow after excision of the fatpad, with the knee in extension (27), they postulated the excision of the fatpad decompressed the patellar in extension allowing for an increased blood flow. The varying techniques and differing equipment used through- out these 3 studies makes direct comparisons im- possible, but do suggest the infrapatellar fatpad makes some contribution to patellar blood flow.
Neural innervation
The nerve supply to the knee has been histori- cally well documented firstly by Garder and latterly Kennedy et al (25,34), with the knee taking innerva- tions from the femoral, obturator and sciatic nerves. The predominant nerve supply to the fatpad is the posterior tibial nerve (34), which provides the ma- jority of fibres to the popliteal plexus ; fibres course from this innervating the posterior capsule, cruci- ates and anteriorly up to the fatpad.
A superfluity of Type VIa free nerve endings has been identified within the fatpad (6,10,24,50) ; these Type VIa free nerve endings can be activated by mechanical deformation or specific immunoreac- tive chemical agents. They initiate afferent signals of pain, pressure and thermal changes to the central nervous system and are active via both fast sharp pain pathways and slow chronic pain pathways (6). Dye et al (22) describe the conscious neuorsensory perception of pain on arthroscopically probing structures of the knee without anaesthetic ; finding the infrapatellar fat pad capable of triggering both severe and localising pain.
Substance-P has been implicated in the genesis of both pain and the induction of a pro-inflammatory response ; several authors have found substance-P nerve fibres in abundance within the fat pad (9,40,55). Furthermore in patients experiencing anterior knee
mace-.indd 95 10/03/16 11:22
96 j. Mace, w. Bhatti, s. anand
Acta Orthopædica Belgica, Vol. 82 - 1 - 2016
pain the number of substance –P nerve fibres within the fatpad increases significantly (40,54). The pres- ence and number of nociceptive afferent immunore- active nerve fibres and ability to release pro-inflam- matory cytokines has led to some question of the fat pad being a potential causative structure in patellar tendinopathy (6,16). There are suggestions it per- forms a modulatory role in the inflammatory path- ways active in osteoarthritis (13) and in an in vitro bovine cartilage model the infrapatellar fat pad tis- sue has been demonstrated to reduce catabolic me- diators (3).
The role of infrapatellar fat pad in biomechanics
In 1950 MacConail surmised the role of fat pads in joints is in occupying anatomical dead space to allow synovial fluid to both circulate around a joint and provide efficient lubrication (41), the rich neuro- vascular supply of the infrapatellar fatpad suggests this may be a simplistic view. A correlation between weight of the infrapatellar fat pad and body height has been demonstrated, perhaps inferring a biome- chanical role within the knee joint (18). Given the rich nociceptive content of the inftapatellar fat pad it comes as no surprise that a form of anterior knee pain can be reliably evoked in normal patients through injection of hypertonic saline into the fat- pad (4,5,28). Patients suffering with anterior knee pain have been demonstrated to show a diminished coordination of motor units between medial and lateral vastus muscle units (44). Hodges et al (28) concluded that after a painful hypertonic saline in- jection to the infrapatellar fat pad there was a sig- nificantly later activation and reduced amplitude of contraction of quadriceps during stair stepping. This deactivation of the quadriceps has been assumed to increase patellofemoral loading leading to an in- creased incidence of patellofemoral cartilage de- generation in some reports of Hoffa’s disease (39).
Bohnsack et al (10), using a cadaveric model, found resection of the infrapatellar fat pad reduced patellofemoral contact pressures, produced a sig- nificant medialisation of the patella and decreased tibial rotation during flexion. Using the same model in a later study Bohnsack et al (8) sought to simulate oedema and mass effect of the fat pad using an in-
flatable fluid cell implanted within the fat pad. An increase in simulated oedema again decreased patel- lofemoral contact area and pressure, whilst increas- ing measured pressure within the fatpad. However, although these cadaveric studies confer a role of the fatpad in biomechanics of the knee, they may not represent true in vivo findings.
Two studies have sought to link the presence of oedema within the fat pad to patellofemoral mal- tracking (33,52), both support the view that oedema within the suprolateral portion of the infrapatellar fat pad is linked with radiological features sugges- tive of patella maltracking, especially an increase in the prevalence of patella alta, the exact cause of this remains unclear.
Diagnosis (inc. prevalence) of fat pad pathology
The diagnosis of infrapatellar fat pad impinge- ment is often considered a rare diagnosis of exclu- sion, or misdiagnosed in cases of recalcitrant ante- rior knee pain (39,43). The population prevalence is not known ; Smillie, in 1962, latterly supported by subsequent authors, separated the rarer primary hy- pertrophy and impingement from the more common secondary disease, occurring in the presence of oth- er pathology such as meniscal tears and ligemen- tous injuries (39,43,51). Kumar et al report a series of 2623 patients undergoing knee arthroscopy, finding its presence as an isolated (primary) lesion in 34 (1.3%) of patients and coexisting with other pathol- ogies as secondary disease in 178 cases (6.8%). Ogilvie-Haris and Giddens report incidence of 1% of patients over a ten year period containing around 1200 patients undergoing surgery.
Several authors have outlined clinical features and diagnostic aids. Anterior knee pain, typically felt in the retropatellar and infrapatellar regions can be considered a universal presentation. There may be an association with patellofemoral crepitus, with some reporting loading the knee and in particular ascending and descending stairs precipitating pain (11,21,38,39,43,47,53).
The importance of excluding causes of other pa- thology by careful examination in particular of the hip and spine should be accepted. Detailed exami- nation of the knee should be performed to locate
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awake patient will not fully extend their knees to the point impingement occurs (39). During a pilot study for ultrasound guided alcohol ablation of the in- frapatellar fat pad House and Connell (30) noted pronounced sonographic anomalies in consisting of hypoechogenicity and neovascularity on colour Doppler. The widespread validity in the use of ul- trasound in diagnosis is still unknown. We have tended to note an relationship of suprolateral fatpad impingement in association with a perceived tight Iliotibial band and through dynamic sonographic assessment have noted a correlation with lateral pa- tella compression syndrome ; but stress that any part of the fatpad can be responsible for impinge- ment. In addition we have observed cases of pre- femoral fatpad impingement presenting with similar symptoms (Fig. 2).
Diagnostic and therapeutic injections of local an- aesthetic and steroid into the fat pad have been rec- ommended by some (21). In these cases it is reported
maximal area of discomfort. The inrapatellar fat pad may appear enlarged and firm in consistency to pal- pation (21,29). Compression of the patellofemoral joint may also cause pain (39,43) Hoffa’s test can be performed and is well described. It is performed with the hips and knees both flexed to 90 degrees ; pressure is then applied to the medial and lateral joint lines. The test is positive for impingement if pain is produced during the last 10 degrees of exten- sion (39,43,47). Kumar et al suggested a modification of this test ; stating the fat pad should not be pal- pated during the test as pressure around the joint could trigger pain in surrounding structures giving rise to false positive results. Instead they recom- mend a passive forced hyperextension by lifting the heel keeping pressure on the anterior tibia, believ- ing if the fat pad is pathological pain will be repro- duced without any direct pressure (39). No figures for the sensitivity or specificity of Hoffa’s test, or its modifications could be found in the construction of this review.
Synovial inflammatory and proliferative causes should be actively excluded through history and the relevant haematological, biochemical and immuno- logical investigations (39,47). Radiographic assess- ment of the knee should be undertaken as needed, the acquisition of basic roentgenographic examina- tion is well supported (23,39,43,47). The role for fur- ther imaging remains controversial. Most authors recommend further imaging, usually in the form of MRI (although CT and radio nucleotide scans have also been advocated (43)). Von Engelhardt et al un- dertook preoperative MRI scans in 62 patients with clinically suspected and arthroscopically confirmed secondary infrapatellar fatpad impingement. Whilst there was no single pathognominic features they found some associations present statistically signifi- cantly more often than in case controlled MRI scans in patients without impingement. These changes in- cluded oedema of the superior/posterior fat pad (present up in up to 48% vs 27% of non impinge- ment control) and the presence of an inflamed in- frapatellar bursa (present in up to 66% of patients with impingement and up to 43% of patients with- out) (53). Kumar et al refute the need for routine MRI scans unless additional pathology is suspected, stating that impingement will not be visible as an
Fig. 1. — T2 weighted MRI scan in coronal plane illustrating typical appearances of suprolateral fatpad oedema from impingement
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Acta Orthopædica Belgica, Vol. 82 - 1 - 2016
incarcerated between the patella tendon and anterior femur, often creating deformation of the tendon in doing so, and again associated with the onset of pain.
Findings at arthroscopy
Aside from Hoffa’s original report in 1902, most authors recommend arthroscopically treating im- pingement of the infrapatellar fat pad. Due to the more common occurrence in conjunction with other pathology (21,39) a careful and thorough arthroscop- ic evaluation of the whole joint should be per- formed. Portals described for adequate visualisation and access are : antrolateral, antromedial, high an- trolateral, high antromedial, midpatella lateral and high antromedial (39,43,47). The typical arthroscopic findings are of a hypertrophic, firm, inflamed fat- pad. Often signs of fibrosis will be present with a whitish covering instead of the normal yellow ap- pearances. There may be adhesions present and vil- lous changes to the overlying synovium (39,43,47,53). The impingement lesion should be visualised with the knee fully extended and the knee deflated (39). Arthroscopic shavers are the instrument of choice for resection, which is carried out cautiously to pro- tect integrity of the patellar tendon. Kumar et al (39) describe three separate arthroscopic morphologies, the first pertained to acute injury related disease with duration of less that 4 months (defined as type 1) ; the fatpad appeared inflamed with contused vil- lous extensions encroaching over the anterior horns of the menisci, engorged blood vessels and signs of recent haemorrhage may be visible. In the more chronic cases (with and without fibrosis – classified as type 2 and 3 respectively) changes included less contusion, but indentation of hyaline cartilage over the femoral condyle at the point of abutment in full extension (type 2), progressing to a firmer fibrotic fatpad with occasional areas of calcification or car- tilaginous degeneration (type 3).
Histology in infrapatellar fat pad impingement
Biopsy at time of surgery is recommended to confirm diagnosis and if there is any doubt of the presence of impingement normal histology should prevent excision (47). Histology will tend to show
an immediate relief of pain and restoration of move- ment. There is a strong consensus of recommending a course of physiotherapy prior to proceeding to op- erative intervention (39,43,47).
Fat pad Dynamics
We believe fat pad impingement to be a dynamic phenomenon that is difficult to fully appreciate through the medium of static imaging. Using high resolution ultrasound we have begun to explore fat- pad kinesiology and have noted varying sites of im- pingement in both knee flexion and extension.
The superolateral portion of the fatpad can be noted to adopt a relaxed, freely expansive and rela- tively fluid state in flexion. On moving to extension the morphology changes as the fatpad becomes in- carcerated by the lateral patella facet and quadriceps tendon ; in symptomatic patients the onset of symp- toms follows this process ; which we have observed to be the most common site of impingement. There may follow an association with the lateral patellar compression syndrome.
The less commonly seen infrapatella fatpad im- pingement follows an opposite pattern, becoming
Fig. 2. — T2 weighted MRI scan in coronal plane illustrating typical appearances of prefemoral fatpad oedema from impingement.
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section on pain after total knee replacement. They found statistically significantly more patients were pain free at 6 months if the fat pad was removed op- posed to retained.
CONCLUSION
Despite being a 110 year old condition the mech- anisms involved in pathogenesis of pain from the infrapatellar fat pad are still largely misunderstood. What is clear from current literature is the complex- ity of the structure and its assessment. If infrapatel- lar impingement is diagnosed correctly, treatment from resection will see improvement in symptoms of pain and function. Further prospective, controlled trials are warranted in the assessment of the roles of surgery, chemical ablation and steroid injections. The indications and value for ultrasonographic dynamic assessment is still in the infancy of wide- spread clinical adoption, but is a technique we have found to be of great assistance and continue to develop.
dilated vessels, synovial hypertrophy, fibrosis or chronic hypertrophy occasionally progressing to calcification or transformation to fibrocartilaginous tissue (21,39,43). Fibrosis and vascular neoformation has been shown to be present in the majority of pa- tients undergoing total knee replacement, with chronic inflammatory changes present in over a third (42), perhaps reflecting the complex involve- ment of the fatpad in the presence of intra articular pathology (13).
RESULTS OF CURRENT STUDIES
There are currently five observational studies without control (level 4 evidence) describing treat- ment of infrapatellar impingement in the recent literature, the findings of these are summarised in table I.
The disparity of data presented from the above studies renders aggregation difficult,…
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